<header>
  <nav>
    <ul>
      <li><a href="">Introduction</a>
        </li>
      <li><a href="">Ancient Times</a></li>
      <li><a href="">Balloon Theory</a></li>
      <li><a href="">First Public Trial</a></li>
      <li><a href="">Experiments</a></li>
    </ul>
  </nav>
</header>



</div>

body {
  padding: 2em;
}

nav ul {
  margin: 0 0 2em 0;
  padding: 0;
  list-style: none;
  display: flex;
  justify-content: space-between;
}

nav li {
  border: 1px solid #ccc;
  border-radius: 5px;
  padding: 5px 20px;
}